Ultraviolet and infra-red detectors are often used as flame detectors in installations where the flame may be the result of an explosion, such as in petro-chemical plants, storage areas for flammable liquids, and the like. Obviously the detector must survive any explosion and provide an alarm signal, and therefore the detector element must be enclosed in an explosion proof housing, and view the area to be protected through an explosion proof window. By enclosing the detector in an explosion proof housing, the detector is permitted by certain industrial requirements to be used in areas in which explosive gases may occur, since the detector has no exposed wiring or electrical apparatus that might produce a spark.
It is also essential that such a detector have means for checking its operability, including the light transmittance of the viewing window and the operability of the circuitry, in response to incident radiation.
Although it is possible to test such detectors by exposing them to an artificial source of radiation of the type that the detector is intended to detect, this method of testing is inconvenient and time-consuming in installations having many detectors, some of which may be relatively inaccessible.
Although external test lamps have been provided for such detectors, such an arrangement does not satisfy the requirements of certain industry specifications, which require that the test lamp be positioned inside the explosion proof housing for the reasons mentioned above.
Systems are known in which the test lamp is positioned inside the housing, and arranged to project radiation out through the viewing window to a reflecting surface, which reflects the test light back through the window and onto the detection cell.
However such systems can give a false indication of inoperativeness, since the test light must pass twice through the viewing window, and must be reflected from a surface which may be covered with an accumulation of dirt, whereas radiation from a fire must pass through the window only once. Although a false signal of inoperability is obviously preferable to a false signal of operability, nevertheless such a system can cause personnel to perform maintenance work on the detector system when it is not needed, and can cause other inconvenience, such as plant shut-down during the period of suspected inoperability of the detector.